Modern aircraft often include an emergency power system to provide power in the event that power is unavailable from the primary or auxiliary power system. Ram air turbines (RATs) are commonly used as an emergency power system to provide electrical and/or hydraulic power in such an event. An RAT presents a rotatable turbine to an oncoming airstream, causing the turbine to rotate. In turn, the turbine then operates a suitable power conversion device (for example, an electrical generator or a hydraulic pump) to provide power to the aircraft.
The RAT is stowed in the aircraft structure, and is deployed manually or automatically by a deployment actuator following the loss of power from the primary or auxiliary power system. The deployment actuator commonly comprises an actuator assembly with a spring-biased mechanism A primary spring member can provide the required force to move the RAT from a stowed position to a deployed position.
As the RAT is deployed during flight, it can experience strong cross winds and turbulence. In order to control the deployment of the RAT, the actuator assembly can further comprise a number of auxiliary springs, which are provided to compensate for the movement of the RAT as it is deployed into the oncoming airstream.
An important design consideration is the time it takes for the RAT to be deployed. The timing must be tightly controlled in order to prevent associated components being damaged during deployment and powering up. One way to improve control over deployment time involves tuning the auxiliary springs to a desired preload or compression level. It is known that shims may be provided to adjust the auxiliary spring preload, for example to take account of machining tolerances or tolerances in the spring itself.
The inventors have identified that current deployment actuators for an RAT could be improved.